This invention relates to diodes and, in particular, photodiodes.
In an optical communication system the sensitivity of the optical receiver is limited by the sensitivity of the photodiode used to convert the lightwaves into electrical signals. The sensitivity of the photodiode is, in turn, determined by its dark current; i.e., the reverse bias leakage current with no light incident on the photodiode. Reverse bias is typically used, rather than the photovoltaic mode, in order to increase the speed of response of the photodiode. High response speed is important in detecting high data rate digital signals. To this end, photodiodes may also be configured as mesas to reduce their capacitance.
At shorter optical wavelengths such as 0.8-0.9 .mu.m, Si photodiodes provide sufficiently low dark currents, but at longer wavelengths such as 1.3-1.5 .mu.m, Si is transparent. Longer wavelengths are of interest because presently available optical fibers exhibit lowest loss and dispersion in this range.
Accordingly, workers in the art have directed their attention to making photodiodes in other materials systems, most notably group III-V compounds such as InGaAs. However, these materials have lower bandgaps than silicon, a feature which is necessary for the detection of longer wavelengths, but which paradoxically increases dark current. Typically, the dark current is about 10 nA for In.sub.0.53 Ga.sub.0.47 As mesa photodiodes of 5.times.10.sup.-4 cm.sup.2 area fabricated by liquid phase epitaxy (LPE) using standard Br-methanol etching techniques to configure the mesa.